1. Field of the Invention
The present invention relates to a multi-layered band pass filter, and more particularly, to a multi-layered band pass filter capable of improving a stop characteristic out of a pass band and reducing the entire size of communication systems.
2. Description of the Related Art
In general, a band pass filter, which is a radio frequency (RF) device that is composed of input/output terminals that play a role in the input/output of a frequency signal; and a combination of a plurality of electrode patterns and a plurality of resonators having frequency selectivity, functions to pass only a frequency signal within a pass band out of frequency signals used for mobile telecommunication systems.
There has been an increasing demand for techniques of densely installing parts inside a substrate due to the increased request of small and thin wireless communication equipment such as portable phones. For this purpose, there has been proposed a method of forming a certain pattern on a multi-layered substrate, the pattern adjusting the coupling to the resonators using transmission lines such as strip lines.
FIG. 1 is a diagram illustrating a configuration of a conventional multi-layered band pass filter. As shown in FIG. 1, a body of the multi-layered band pass filter is manufactured by laminating a plurality of dielectric layers 1-5 formed of ceramic dielectric materials. Here, a first ground plane 6 and a second ground plane 13 are formed respectively on a first dielectric layer 1 and a fifth dielectric layer 5; an electrode pattern 11 coupled in parallel to resonators to form a coupling capacitor CC and electrode patterns 12a and 12b formed between the resonators and a grounding conductor to form a load capacitor CL are printed as a thick film on the second dielectric layer 2; first and second strip line resonators 10a and 10b are printed as a thick film on the third dielectric layer 3; and electrode patterns 8a and 8b formed between the resonators and the input/output terminals to form an input/output coupling capacitor C01 and electrode patterns 9a and 9b formed between the resonators and a grounding conductor to form a load capacitor CL are printed on the fourth dielectric layer 4.
However, the resonators used in the conventional multi-layered band pass filter have a problem that the total volume of the filter is increased as their frequencies become low because the resonators are in the straight-line form having a constant width and their entire λ/4 length is used.
Also, since the input/output terminals have the same shape as the capacitor and are coupled to the resonators, an insertion loss of the filter is high in the multi-layered structure due to the change in process, for example the dimensional difference between an upper layer and a lower layer or the positional changes, which leads to the deteriorated performance of wireless telecommunication systems.